1. Field of the Invention
The present invention relates to a main valve mounted at an opening for connecting a vacuum chamber and a main vacuum pump, and more particularly, to a main valve capable of finely adjusting the conductance of exhaust of equipment used in manufacturing fiat panel displays, recording media, and semiconductors (or integrated circuits).
2. Description of the Related Art
In a vacuum processing apparatus, such as a sputtering apparatus, a dry-etching apparatus or a plasma enhanced CVD apparatus, various kinds of gases (referred to hereinafter as "process gas"), such as for example argon gas for plasma discharge, are introduced into a vacuum chamber to give a surface treatment to a substrate. The pressure of the process gas in the vacuum chamber is controlled in order to properly treat the surface of the substrate. Controlling the pressure of the process gas is, as is well known, performed by (a) adjusting the flow rate of the introduced gas, (b) adjusting the conductance of a variable orifice (e.g., a conductance adjusting mechanism) located between a main valve of the vacuum chamber and a main vacuum pump, or (c) stepwise stopping between an opening position and a closing position a valve seat of the main valve through the use of an air cylinder.
FIG. 1 illustrates a conventional bridge-type main valve, which is widely used as a main valve, located at an outlet 5 to which a vacuum chamber 4 and a main vacuum pump 3 are connected. In the bridge-type main valve, as shown in FIG. 1, a valve seat 2 is connected to the top ends of the shafts 14 of two air cylinders 1a, 1b. The air cylinders are on opposite ends of the outlet 5 at the bottom wall of the vacuum chamber 4. The valve seat 2 moves upward and downward by shifting the shafts 14 of the air cylinders 1a, 1b in the vertical direction. The valve seat is set at an open position 7 during vacuum evacuation in the vacuum chamber 4 and at a closed position 6 when the vacuum evacuation is stopped. Referring to FIG. 1, a variable orifice 8 is interposed between the outlet 5 and the main vacuum pump 3. The exhaust conductance of the process gas is controlled by adjusting the degree of opening of the variable orifice 8. The pressure of the process gas in the vacuum chamber 4 may be adjusted by this control of the exhaust conductance.
If a variable orifice is not mounted on the main valve, the pressure in the vacuum chamber can be adjusted only by control of the flow rate. If the pressure in the vacuum chamber is lower than the desired pressure, the flow rate of the process gas has to exceed the pumping ability of the main vacuum pump. The desired pressure, however, sometimes cannot be obtained using flow rate control because of an insufficient flow rate of the process gas.
If the pressure is adjusted by the variable orifice 8, this problem can usually be solved. However, even if the variable orifice 8 is fully opened, the maximum pumping speed of the main vacuum pump 3 is sacrificed (i.e. lowered) by the inherent conductance of the variable orifice 8.
Further, the shafts 14 of the air cylinders 1a, 1b operate rapidly through the use of compressed air. Therefore, the valve seat 2 connected to the shafts 14 also moves rapidly from the open position 7 to the closed position 6. The rapid movement of the valve seat 2 makes it possible to easily achieve a speedy start and a quick stop of the evacuating operation. However, since the shafts 14 move by compressed air, the valve seat 2 can stop only at the open position 7 and the dosed position 6.
FIG. 2 shows a main valve having a stepwise stop mechanism which allows the valve seat 2 to stop at the dosed position 6, the open position 7, and a stepwise stop position 9. In the stepwise stop mechanism of FIG. 2, a plurality of spacers 40 are attached to the leading end of the shaft 14 projecting from each of air cylinders 1a, 1b. The movement of the valve seat 2 becomes stepwise by removing one or more spacers 40. The exhaust conductance can be adjusted by this stepwise stop of the valve seat 2.
However, since the stop position 9 of the valve seat 2 is adjusted stepwise by removing the spacers 40, the exhaust conductance cannot be finely controlled, and therefore, the pressure of process gas in a vacuum chamber also cannot be finely controlled. For example, in a reactive sputtering method, in which the composition ratio of a film varies with sensitivity to the flow rate ratio (partial pressure ratio) of a plurality of process gases, if the pressure in the vacuum chamber is controlled by this stepwise stop mechanism, a thin film having a desired composition ratio sometimes may not be obtained.
As described above, a conventional bridge-type main valve is disadvantageous in that the pumping speed is lowered if pressure is controlled by a variable orifice located between a main valve and a main vacuum pump. Further, fine adjustments to the exhaust conductance are difficult in a mechanism which stops a valve seat stepwise.